1. Field of the Invention:
This invention relates to the optical recording art and more particularly, to novel photochromic compounds. Also, it relates to photosensitive compositions and materials comprising such compounds.
2. Description of the Prior Art:
Photochromic photosensitive spiropyran materials are known to have a number of advantages: (1) high resolution is obtained without involving any particulate property; (2) no specific developing and fixing treatments are needed; (3) thin film formation is possible because of high intensity of formed color; and (4) erasion and rewriting are possible. Accordingly, attempts have been made to apply these materials to various recording and memory materials and also to duplicating materials. Especially in recent years, application of these materials as recording media, e.g. optical video discs, in which a laser beam is used to record and reproduce information signals while making use of the features (1) and (4) indicated above, is highly expected.
Known photochromic, photosensitive spiropyran materials have an absorption wavelength, in colored state, ranging at most from 400 nm to 700 nm. For laser beam recording or reproducing purposes, it is essential to use a gas laser such as Ar.sup.+ or He--Ne laser. As is well known, small-size, light-weight semiconductor lasers have recently been developed markedly. So, semiconductor lasers tend to take the place of gas lasers for recording and reproducing purposes.
Semiconductor lasers which are actually applied for laser recording or reproducing operations are those whose emitting wavelength ranges from 780 to 850 nm. At present, semiconductor lasers which emit at shorter wavelengths have been extensively developed. It is considered that there is the high possibility of putting into practice semiconductor lasers emitting at about 700 nm in a very near future (see, for example, "Present State and Future of Semiconductor Laser Recording", Journal of Japanese Photographic Association 44 (2) (1981), by Fujitaro Saito, and "Development and Present State of Visible Light Semiconductor Lasers", Optlonics (No. 9), 41 (1982).
In order to apply photochromic photosensitive spiropyran materials as a recording medium for semiconductor laser recording and reproducing purposes, the materials should have good absorption characteristics with respect to light having a wavelength higher than as in prior art cases and particularly a wavelength higher than 700 nm.
As is known in the art, the following spirothiopyran compound has a good absorption characteristic, in colored state, against light having such a range of wavelength as mentioned above (see J. Phys. Chem. 72, 997 (1981), by H. S. Becker and J. Kolc). ##STR2##
Although this compound has a high absorption characteristic in colored state with respect to light having a wavelength ranging from 600 to 850 nm when placed in a 3-methylpentane solution at 77.degree. K, it does not form any color at a normal temperature even in solution or polymer film. The compound is able to develop a color only at temperatures lower than 0.degree. C. and the once developed color disappears immediately after returning to a normal temperature. Thus, this compound cannot be practically used.